1. Field of the Invention
The present invention relates to a method for forming electrodes with high adhesion to diamond for diamond electronic devices such as heat sinks, diodes, and transistors.
2. Description of the Related Art
Diamond has high hardness and stability against heat, radiation, and chemicals. It also has a large band gap of 5.4 eV. Diamond is electrically insulating, but becomes semiconducting by doping. Thus, diamond is expected to be used for electronic devices which can be operated at high temperatures. Furthermore, diamond is transparent over a wide wavelength range between ultraviolet and infrared, and is used for optical windows.
Diamond films formed by vapor deposition methods also have the above-mentioned excellent characteristics, and are used for coatings on cutting tools and speaker diaphragms, as well as for electronic applications such as heat sinks, diodes, and transistors.
It is well known that diamond film can be formed by vapor phase synthesis using plasma chemical vapor deposition (CVD) at low cost. Similarly for bulk diamonds, synthesized diamond film becomes semiconducting by doping with an impurity such as boron (B).
In the fabrication of electronic devices using diamond, it is necessary to form electrodes on the surface of diamond. Known methods of forming electrodes on the surface of diamond are as follows:
(I) Ohmic contacts were obtained by a method of vapor-deposition of metals liable to form carbides, e.g. Ta, on diamond followed by a heat treatment by electron beam irradiation, thus forming a carbide intermediate layer at the interface between diamond and the metal electrode, thereby enhancing the adhesion A. T. Collons et al. Formation of electrical contacts on insulating and semiconducting diamonds, Diamond Research, 1970!. This method will be referred to hereafter as Method I.
(II) Ohmic contacts were obtained by a method of vapor-deposition of metals liable to form a carbide, e.g. Mo, on diamond followed by a heat treatment using a vacuum furnace K. L. Moazed et al. Material Research Society Symposium Proceedings 162, P.347, 1990!.
Using this method, electrodes with high adhesion were formed by heat-treating metal films having multiple layer structures such as Au/Pt/Ti, Au/Ti, or the like. This method will be referred to hereafter as Method II.
(III) Metal electrodes were formed after irradiating Ar ions on the surface of diamond, thereby transforming the surface thereof into graphite C. B. Child, Fourth Annual SD 10/IST-ONR Diamond Technology Initiative Symposium, 1989!. By this method, Au electrodes with high adhesion were formed on the surface of diamond. This method will be referred to hereafter as Method III.
However, the above-described methods have the following problems:
In Method I, it is necessary to perform the heat treatment using an electron beam generator at high vacuum, which is expensive and time consuming. Thus, this method is disadvantageous in terms of production cost.
In Method II, the metal electrodes which have been vapor-deposited on diamond must be heated at high temperature (e.g. 800.degree. C.) to form a carbide. This causes a stress at the interface between the electrode and the diamond due to the difference in thermal expansion coefficients. Thus, this method has the disadvantage that the electrodes are thermally damaged and tend to peel off from the diamond.
Also, in a heat treatment at high temperatures, the surface of a diamond is transformed into graphite. Thus, in this method, low resistance parasitic channels are liable to be formed on the surface of a diamond. Furthermore, similarly for Method I, since this method employs a vacuum furnace in the heat treatment, this process is very time consuming.
In Method III, an expensive ion beam generator has to be operated at high vacuum, which is costly and time consuming. Furthermore, when a processed sample is subjected to ultrasonic cleaning, the electrodes partially or totally peel off because of poor adhesion.